Chemical fertilizers originally were produced as substitutes for manure. The nature of fertilizer products and their production methods, however, have changed drastically with the development of modern agricultural practices. The goal for modern farmers is to obtain maximum yields of plant produce per unit area of ground. Included in the trend toward maximum yields are the planting of the same crops year after year on the same ground, growing a larger number of individual plants per unit area and the use of new plant varieties bred specifically for crowded planting for maximum yield. For this intensive type of agriculture, chemical fertilizer has to be applied at very high rates and in such a scheme so as to supply all plant nutrients in a form which is readily available to the plants. Moreover, when chemical fertilizer is applied at high rates, it is important to prevent accumulation in the soil of undesirable ingredients, such as chloride ion and sulphate ion. A low-chloride chemical fertilizer also is necessary for application to chloride-sensitive crops, such as tobacco, potatoes and grapes, as well as for plant germination or foliage spraying.
Potassium phosphates have been long recognized as a potentially attractive fertilizer product because of their high nutrient content, their chloride-free advantage over conventional NPK fertilizers, their slow release characteristics and excellent agronomic properties in general. Shipping and storage costs for high nutrient (P.sub.2 O.sub.5, K.sub.2 O) potassium phosphates (80-85% nutrient value for monopotassium phosphate and 90-100% nutrient value for potassium polyphosphates) would be significantly lower than for their lower nutrient value competitive products. In addition, being a chloride-free fertilizer, potassium phosphates may replace fertilizer in areas where chloride build-up in the soil or plant sensitivity to chlorides is a concern. Agronomic studies have confirmed that potassium phosphates are high quality, slow release fertilizers.
While technologies for the production of potassium phosphate do exist, (including neutralization of H.sub.3 PO.sub.4, acidulation of KCl with excess H.sub.3 PO.sub.4, high temperature and pressure fusion of KCl and H.sub.3 PO.sub.4 and ion-exchange and/or solvent extraction of KCl and H.sub.3 PO.sub.4), the commercial manufacture of this valuable fertilizer in large quantities has not yet been realized, chiefly because of economic reasons. Limited quantities have been produced by direct neutralization of phosphoric acid with caustic potash for specific applications where a high sale price justified the high cost of production, such as in the greenhouse and hydroponics industry, seed germination and plant foliage spray. The high cost of caustic potash coupled with its corrosive nature and difficulty in storage have prompted many researchers and chemists to look for a process to replace potassium hydroxide with a much cheaper source of potassium, such as potassium chloride.
Potassium phosphate can be produced by the reaction of phosphoric acid with potassium chloride, according to the well known reaction: EQU H.sub.3 PO.sub.4 +KCl.fwdarw.KH.sub.2 PO.sub.4 +HCl
In practice, the removal of hydrogen chloride from the reaction mixture is difficult to achieve, unless a high temperature and/or a large excess of phosphoric acid are employed. In the first case, the water-insoluble potassium metaphosphate (KPO.sub.3).sub.n is preferentially formed and, in the second case, separation of pure potassium salt from the excess acid is an impractical as well as expensive unit operation.
A number of workers have described specific methods for the removal of chloride from the reaction product of potassium chloride with phosphoric acid. Ross and Hazen in U.S. Pat. No. 1,456,831 teach the reaction of potassium chloride with phosphoric acid at temperatures of 250.degree. C. and above and describe the use of air to increase the rate of HCl evolution. Askenasy et al, Zeit. Anorg. u. Allgem. Chemie, 189,305-28 (1930), discuss the use of steam in place of air for increasing the rate of reaction of potassium chloride with phosphoric acid and indicate that, even at 200.degree. C., a residual chloride content of 4% Cl is obtained in the product.
Britzke et al., J. Chem. Ind. (Moscow), 7, 4-11 (1930), describe experiments similar to those of Askenasy et al. as well as the use of reduced pressure in accelerating the reaction of potassium chloride with phosphoric acid. The use of reduced pressure also is mentioned by Kaselitz, U.S. Pat. No. 1,805,873. More recently Provoost, French Patent 1,395,837 has described the preparation of a specific fertilizer having a P.sub.2 O.sub.5 to K.sub.2 O weight ratio of 1:1 by addition of one mole of sulfuric acid for each two moles of phosphoric acid used in the reaction. Provoost also describes the use of air to increase the rate of reaction.
The prior art, therefore, has disclosed that the reaction of potassium chloride with phosphoric acid should be carried out at temperatures higher than about 200.degree. C. and/or in the presence of excess phosphoric acid. Even under such stringent operating conditions, the chloride content in the resulting reaction product could not be reduced further than 4 wt. % chloride. However, to be universally acceptable as a low-chloride fertilizer, the finished product should contain 2 wt. % or less of chloride, as required for certain crops or applications. Moreover, to be economical, the process should be carried out at a temperature below 200.degree. C. for three reasons. First, at temperatures near or above 200.degree. C., the cost of suitable materials for construction increases to such an extent that the process cannot be operated economically. Secondly, impurities normally associated with fertilizer-grade phosphoric acid, particularly iron and magnesium, react with the phosphorus and potassium components of the system above 200.degree. C. to form complex materials which may be unavailable for utilization by plants. Thirdly, above about 200.degree. C., organic materials associated with the commonly-used phosphoric acid is evolved, contaminating the by-product HCl and rendered it unsuitable for sale.